Biopotential amplifiers are essential building blocks in multiple types of biological signal sensing or recording systems. For instance, neural signal amplifiers or neural amplifiers are essential building blocks of neural signal sensing or recording systems. Ideally, a neural signal amplifier should exhibit high input impedance, low power consumption, low Noise Efficiency Factor (NEF) concurrent with low Power Efficiency Factor (PEF), and minimal sensitivity to input referred noise. Furthermore, a neural signal amplifier should be able to significantly reject common-mode interference (CMI). Sources of strong CMI include AC mains 50/60 Hz interference, and possibly muscle electromyographic (EMG) artifacts.
FIG. 1A is a schematic illustration showing portions of an embodiment of a representative conventional multi-channel neural signal amplifier 1, which includes a front end, front end circuitry, or an input stage 10; and a second stage or second stage circuitry 40. The front end 10 includes a low noise amplifier (LNA) corresponding to each neural signal recording channel. Each of such LNAs has an input that is coupled to a neural signal recording electrode by which the recording channel LNA can receive an input neural signal, and hence each of such LNAs can be referred to as a signal LNA. The front end 10 also includes a reference LNA corresponding to a reference channel, where the reference LNA includes an input to which a reference signal can be provided by way of a reference electrode. The second stage 40 includes a second stage amplifier G2 corresponding to each channel, which has a signal input (e.g., its non-inverting input as per FIG. 1A) coupled to an output of a counterpart front end signal LNA, and which has a reference input (e.g., its inverting input as per FIG. 1A) coupled to the output of the reference LNA. Thus, for a neural signal amplifier having N channels, the output of the reference LNA is replicated across the reference inputs of N second stage amplifiers G2. As a result, the representative neural signal amplifier 1 of FIG. 1A can be referred to as having a replica reference topology.
For any given channel 1 . . . N (e.g., N=16 in a representative implementation), the second stage amplifier G2 performs a differential signal subtraction between the input neural signal for that channel and the reference signal, and provides a corresponding channel output voltage Vout 1 . . . N. As will be readily understood by individuals having ordinary skill in the relevant art, prior to the differential signal subtraction by each second stage amplifier G2, each signal LNA amplifies both input neural signals and CMI signals. If the CMI signals are large, the signal LNAs can easily become saturated, thereby disrupting any ongoing neural signal amplification, for instance, in a manner indicated in FIG. 1B, which shows representative neural input signals, saturated signals output by the signal LNAs, and saturated signals output by the second stage amplifiers G2. This saturation problem can be particularly significant for modern neural signal amplifiers that operate at low supply voltages, and which require front end LNAs having high gain. For a representative conventional neural signal amplifier 1 having front end LNAs designed to accommodate input signals having a maximum input peak-to-peak amplitude of 12 mV to produce a maximum output voltage of 250 mV peak-to-peak, as shown in FIG. 1C, in the presence of CMI signals having an amplitude of up to 100 mV peak-to-peak, the front end LNAs can easily saturate.
A common mode feedback (CMFB) technique can be employed to reduce the gain of the front end LNAs and hence enhance the input common mode swing. More specifically, as illustrated in FIG. 1D, a CMFB signal generator 50 having a summing amplifier can be added to the conventional neural signal amplifier 1 of FIG. 1A to produce a modified neural signal amplifier 5 that provides a CMFB signal to a non-inverting terminal of each front end LNA, while input neural signals are provided to an inverting terminal of each signal LNA and the reference signal is provided to an inverting terminal of the reference LNA. FIG. 1D also illustrates representative signal flow through the modified neural signal amplifier 5 in the presence of input CMI signals. As a result of the CMFB, the front end LNAs attenuate CMI signals, common mode gain is reduced, and intrinsic CMRR (ICMRR) is increased. FIG. 1E illustrates representative signal flow through the modified neural signal amplifier 5 when a neural signal having a nonzero amplitude is applied to the input of one of the signal LNAs, while the other signal LNAs receive zero amplitude signals at their inputs. FIG. 1F illustrates representative signal flow through the modified neural signal amplifier 5 in the presence of input referred noise, showing that the CMFB signal generator 50 does not contribute to input referred noise.
In spite of its advantages, this common mode feedback technique requires that each front end LNA of the modified neural signal amplifier 5 be a differential amplifier, which unfortunately requires an extra input terminal for each front end LNA, and which undesirably results in additional power consumption and larger circuit area.
In addition to the foregoing, in the representative conventional neural signal amplifier 1 of FIG. 1A as well as the modified neural signal amplifier 5 of FIG. 1D, the reference LNA drives the reference inputs across all second stage amplifiers G2, while each signal LNA drives the input capacitance of only its corresponding second stage amplifier G2. The output of the reference LNA thus encounters a different load capacitance than the output of each signal LNA.
FIG. 1G illustrates this load capacitance mismatch situation for the conventional neural signal amplifier 1 of FIG. 1A, showing that for N channels, the reference LNA drives a load capacitance that is N times higher than the load capacitance driven by any given signal LNA. More particularly, for N input channels, the total effective input capacitance seen at the reference LNA input is given by
                                          C                          LNA              ⁢              _              ⁢              REF                                =                                                    N                ·                                  C                  1                                ·                                  (                                      C                    fb                                    )                                                                              C                  1                                +                                  C                  fb                                                      ≈                                          N                ·                                  (                                      C                    fb                                    )                                            ⁢                                                          ⁢              for              ⁢                                                          ⁢                              C                1                                                    >>                  C          fb                                    (        1        )            
When all front end LNAs are driven by a CMI signal, the effective input capacitance seen at each signal LNA input is derived to be
                                          C                          LNA              ⁢              _              ⁢              SIG                                =                                                                      C                  1                                ·                                  (                                      C                    fb                                    )                                                                              C                  1                                +                                  C                  fb                                                      ≈                                          C                fb                            ⁢                                                          ⁢              for              ⁢                                                          ⁢                              C                1                                                    >>                  C          fb                                    (        2        )            and thus for N>=2, the output of the reference LNA encounters a larger load capacitance than that encountered by the output of each signal LNA.
Further to the foregoing, the front end LNAs are usually designed using operational transconductance amplifiers (OTAs) that intrinsically have high output impedance, as they have to be biased with low biasing current to keep the total power consumption low. This high output impedance in conjunction with the load capacitance presented by the input capacitances of the second stage amplifier G2 forms a pole which determines the high side 3-dB cutoff frequency, fu. For higher N, the reference LNA's fu is markedly lower than that of the signal LNAs. This means the phase response of the reference LNA and the signal LNAs are markedly different. When a common mode signal appears at all front end LNA inputs, the signals output by the front end LNAs and appearing at the two inputs of the second stage amplifier G2 have non-zero phase difference for frequencies above DC, for instance, in a manner representatively illustrated in FIG. 1H.
The second stage amplifier G2 amplifies this phase difference and essentially performs a differential phase-to-amplitude conversion, which is a form of common mode-to-differential signal conversion that increases with increasing signal frequency, for instance, in a manner shown in FIG. 1I. Thus, the phase difference arising from the difference in capacitive loading at the output of the reference LNA versus that at the output of each signal LNA undesirably increases common mode gain, which degrades the intrinsic common mode rejection ratio (ICMRR) as input signal frequency increases (i.e., this phase difference causes ICMRR to decrease with increasing signal frequency).
A need exists for a neural signal amplifier system that overcomes the foregoing problems.